1. Field of the Invention
The present invention relates to an electroplating apparatus and more specifically to a structure of an electroplating apparatus for forming a metal film on a substrate.
2. Description of the Background Art
The principle of electroplating lies in that a voltage is applied to a cathode and an anode to deposit a metal from an electrolyte on the cathode side. In the reaction on the cathode side, electrons are supplied from the electrode to metal ions in the electrolyte, whereby the metal is deposited. The reaction on the anode side can be roughly classified as one of two kinds, according to the anode material.
If the metal to be deposited on the substrate is used as the anode material, the metal of the anode releases electrons to cause reaction in which the ions are eluted into the electrolyte. Such an anode is referred to as a soluble anode. On the other hand, if a metal nobler than the metal to be deposited on the substrate is used as the anode material, hydroxide ions (OHxe2x80x94) in the electrolyte release electrons at the anode to cause a reaction in which water and oxygen are generated. Such an anode is referred to as an insoluble anode.
An electroplating method using a soluble anode has an advantage in that the amount of metal ions in the electrolyte can be maintained constant because the same amount of metal as the metal deposited on the substrate is supplied from the soluble anode to the electrolyte.
FIG. 16 is a cross section view illustrating a structure of a Cu electroplating apparatus using a soluble anode, which is shown in Proc. of 1993 VLSI Multilevel Interconnection Conference, pp. 470-477. Referring to FIG. 16, this Cu electroplating apparatus includes a plating tank 1, a soluble anode 2, an electrolyte 3, a conductive layer 5 deposited on a substrate 4, a substrate holder 6 for holding the substrate 4, a contact electrode 7, an electrolyte inlet 8 having an opening end 8a, and an electrolyte outlet 9. The electrolyte 3 is introduced from the electrolyte inlet tube 8 into the plating tank 1, and is discharged from the electrolyte outlet 9 by overflowing. The soluble anode 2 is disposed in the electrolyte 3, and the substrate 4 fixed to the substrate holder 6 is disposed at an upper part opposite to the soluble anode 2. By thus disposing the substrate 4 at an upper part of the plating tank 1, the substrate 4 can be easily taken in and out of the electrolyte tank 1 without discharging the electrolyte.
An electric current must be passed in order to perform electroplating. For this purpose, the conductive layer 5, referred to as a seed layer, is formed on the substrate 4. In this structure, the electric current is supplied from the contact electrode 7 to the conductive layer 5, whereby the electroplating is performed. Further, in this Cu electroplating apparatus, a mechanism for rotating the substrate holder 6 can be adopted to produce a better film thickness distribution.
However, according to the method using this Cu electroplating apparatus, the volume of the soluble anode 2 decreases as the electroplating is performed, changing the distance between the soluble anode 2 and the cathode. This results in a change in the distribution of the thickness of the formed film or in the film quality.
Further, according to the plating method using this Cu electroplating apparatus, it is necessary to form a coating called a black film on the surface of the soluble anode 2 in order to carry out the elution of the soluble anode 2 smoothly. This black film includes an oxide of phosphorus, an oxide of copper, or the like added to the soluble anode 2. Since the adhesion strength of the black film is extremely weak, the black film disadvantageously causes particles in the electrolyte 3.
On the other hand, by an electroplating method using an insoluble anode, the anode is not eluted, so that the aforesaid problem is not raised. However, one of the problems involved in the electroplating method using an insoluble anode is generation of oxygen at the anode. If the substrate 4 is disposed above the anode in the same manner as in the electroplating apparatus using a soluble anode, the generated oxygen is accumulated on the surface of the substrate 4, thereby obstructing deposition of the metal on the substrate surface. In particular, if the substrate surface has irregularities, oxygen is accumulated in the recessed part to obstruct the deposition of the metal in the recessed part of the substrate surface, so that the recessed part is poorly filled.
Hitherto, in order to prevent such a problem, a construction has been adopted in which the substrate 4 is disposed below the anode in a conventional electroplating apparatus using an insoluble anode. FIG. 17 is a view illustrating a structure of a conventional Cu electroplating apparatus disclosed in Japanese Patent Laid -Open No. Hei. 06-280098. This Cu electroplating apparatus includes a plating tank 1, an electrolyte 3, a substrate 4, a conductive layer 5 on the substrate 4, a substrate holder 6, a contact electrode 7, an electrolyte inlet tube 8 having an opening end 8a, an electrolyte outlet 9, an insoluble anode 10, and a seal 11.
As shown in FIG. 17, in the conventional electroplating apparatus using an insoluble anode, a construction is adopted in which the substrate 4 is disposed below the anode. Therefore, the electrolyte 3 in the plating tank 1 must be discharged in exchanging the substrate 4. This results in a problem of long period of time for performing a plating process on the substrate 4.
Further, a small amount of the electrolyte 3 remains in the plating tank 1, making it difficult to control the amount of the electrolyte. Also, the residual electrolyte 3 adheres to the seal 11, and the electrolyte 3 adheres onto the surface of the conductive layer 5 on which the metal film is to be formed next, corroding the conductive layer 5. Furthermore, corrosion of the conductive layer 5 causes poor contact between the contact electrode 7 and the conductive layer 5. Also, the residual electrolyte 3 drips down on the outside of the plating tank 1, corroding wiring and other parts of the electroplating apparatus.
Further, since the substrate holder 6 cannot be rotated due to structural reasons, there will be a problem of poor film thickness distribution.
The object of the present invention is to provide an electroplating apparatus having a structure in which the cathode is disposed above the anode in the electroplating apparatus using an insoluble anode.
The electroplating apparatus according to the present invention is an electroplating apparatus in which a plating tank is filled with an electrolyte and a voltage is applied between a cathode and an anode disposed in the plating tank to form a metal film on a substrate on a cathode side, the electroplating apparatus including: an anode made of an insoluble material that is not eluted into the electrolyte at the time of forming the metal film; a cathode disposed above the anode; and a means disposed between the anode and the cathode for preventing oxygen generated at the time of forming the metal film, from reaching the substrate.
Disposal of such a means can prevent oxygen generated at the anode from reaching the cathode. As a result of this, deposition of metal on the cathode surface can be prevented from being obstructed by accumulation of generated oxygen on the cathode surface. In particular, this effect is great if the cathode surface has irregularities. This can provide a good thickness distribution of the film formed on the cathode.
Further, in order to realize the present invention in a more preferable state, a mesh filter is disposed between the anode and the cathode. Furthermore, in order to remove oxygen with certainty, the filter is disposed to include the anode in a plan view.
Here, if oxygen is accumulated on the filter, the electric field distribution and the electrolyte flow are disturbed to cause non-uniform thickness distribution of the formed film and poor reproducibility of film forming. In order to avoid such a state, the following modes are adopted.
As a preferable mode of the invention, the filter has a shape which is sloped upwards as viewed from a central part to an outer peripheral part of the filter. By adopting this shape, the oxygen that has reached the filter can be smoothly guided upwards.
Further, as a preferable mode of the invention, the filter has one or more openings circumferentially disposed in a vicinity of an outer peripheral part. Adoption of this configuration makes it possible to allow oxygen to escape from the openings disposed in the outer peripheral part of the filter. As a result of this, oxygen escapes outward through the outside of the cathode, so that the oxygen can be allowed to escape without reaching the cathode.
Further, as a preferable mode of the invention, the electroplating apparatus further includes an electrolyte inlet tube in the plating tank for introducing the electrolyte; the electrolyte inlet tube penetrates through a central part of the anode; and an opening end of the electrolyte inlet tube is disposed on an upper surface side of the anode. Further, as a more preferable mode of the invention, a lower outlet for letting the electrolyte out is disposed on a side surface of the electrolyte inlet tube positioned below the anode. Adoption of this construction makes it possible to form a flow of the electrolyte oriented from the central part towards the outer peripheral part of the filter, whereby the oxygen that has reached the filter can be smoothly guided to the openings.
Further, as a preferable mode of the invention, an opening is disposed at a central part of the filter. Adoption of this construction makes it easier to control the flow of the electrolyte, whereby a more uniform film thickness distribution can be obtained.
Further, as a preferable mode of the invention, the opening end of the electrolyte inlet tube is disposed to be in communication with the opening disposed at the central part of the filter. Furthermore, as a more preferable mode, the electrolyte inlet tube is disposed to extend to the opening. Furthermore, as a more preferable mode, an upper outlet for letting the electrolyte out is disposed on a side surface of the electrolyte inlet tube positioned between the filter and the anode. Further, as a preferable mode of the invention, the filter includes a hanging part that allows communication between the opening of the filter and the opening end of the electrolyte inlet tube. By this construction, the filter can be used even if the mechanical strength of the filter is small. Also, by disposing an upper outlet or lower and upper outlets, it is possible to make a flow of the electrolyte along the side surface of the plating tank 1.
Further, as a preferable mode of the invention, an outer peripheral part of the filter is connected to a lower end of a cylindrical member disposed in an inside of the plating tank. By this construction, an oxygen outlet is formed between the plating tank and the cylindrical member, whereby the oxygen captured by the filter can be discharged to the outside with certainty through this oxygen outlet. Therefore, the captured oxygen can be prevented from returning to the cathode side again.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.